Perimetric methods are regularly used to determine the visual field of an eye, and, accordingly, of the person being examined. An eye's visual field may be limited as a result of a disease of the eye, for example, glaucoma, wherein the glaucoma may progress to complete loss of vision. The cause of vision loss may also be a disease of the optic nerve or damage to the nerve fibres (nerve fibers). The nerve fibres in the eye may be divided into various nerve fibre regions of a retina, all of which lead to the optic nerve (optic disc) and the “blind spot”, where no photoreceptor cells are located.
In a perimetric examination of an eye's visual field, the patient's eye is exposed to optical stimuli, and the patient signals whether the given stimulus was detected or not. Thus, perimeters are known in which a patient looks into a hemisphere, via which the points of light are projected in different positions and at various levels of brightness under the control of a computer program. In this way, it is possible to examine both the overall extent of a visual field and the condition of selected regions thereof. The brightness of the points of light may be varied in such manner that a threshold value above which the patient detects the point of light can be determined for a position on the retina.
Various methods for determining visual fields and threshold values are known from the related art. In essence, a series of stimuli with various gradations of brightness are directed at a point on the retina to be measured or tested, so that the specific threshold value for the point may be determined. In order to localise a threshold value, this may be carried out with uniform or non-uniform gradations, for example. Additionally, a data base containing representative thresholds for a number of persons may be incorporated. In this way, for example, a possible threshold value may be delimited within a threshold range taking into account a patient's age and sex. It is also known that there is a relationship between directly adjacent regions or measurement points in the retina. Thus, for example, threshold values typically do not differ substantially between neighbouring points. Accordingly, if a threshold value has been determined for a first point, a threshold value for a directly adjacent point may be sought within a probable threshold value range. Accordingly, the overall advantage of such a method is that a patient has to be exposed to fewer stimuli and a visual field may be determined more quickly.
It should also be noted that a large number of stimuli does not necessarily result in more accurate results when determining a visual field, since it is quite normal for persons undergoing examination to feel the effects of neurological retinal fatigue or tiredness while their visual field is being measured, thereby distorting the results of the examination. Accordingly, several tests or measurements of a visual field may also be carried out at different times in order to counter the effects of fatigue. Consequently, it is generally desirable to determine a visual field using a number of stimuli that enables the time for measuring the visual field to be kept as brief as possible, so that the effect of fatigue does not cause any significant distortion of a measurement result. At the same time, the requirement exists to measure a visual field as accurately as possible, and for this a large number of stimuli is necessary.
The underlying object of the present invention is, therefore, to provide a perimetric method for measuring the visual field of an eye that yields sufficiently accurate measurement results with a relatively small number of stimuli.